1. Field of the Invention
This invention pertains to the field of control units for rotatable shaft-actuated devices such as valves. More particularly, it relates to control units to reversibly operate valves between their full-open and full-closed positions.
2. Description of the Prior Art
Many processes involve valves and other rotatable shaft-actuated devices that need to be opened, closed and otherwise adjusted. In complicated processes involving numerous steps and equipment, so many valves may be involved that manual control is too difficult. Accordingly, there has been developed a series of devices known as "motorized controllers" that are attached to these devices to operate them, i.e., in the case of valves, to rotate the valve stems through electric motors and other associated components. These controllers drive the valves usually between fully-opened and fully-closed positions.
Three problems plague this industry. The first involves frictional buildup in the valve. While the valve stem is in motion, there is generally constant friction encountered in the valve stem and the load on the control unit drive remains relatively uniform. That is to say, there is no buildup of forces in the valve itself and the movement from full-open to full-closed position may be handled by the controller without difficulty. When, however, the valve reaches the fully closed position, a sudden increase in frictional load occurs in the valve stem because of the tightness achieved between the valve parts during opening or closure as well as some friction buildup caused by flow interruption in the line. To open a fully-closed valve therefore requires the controller to initially overcome this rather large frictional force. Once the valve is cracked open by the controller, the stem friction drops to the relatively low value throughout the remainder of valve travel. This high initial frictional load often causes disruption of movement in the valve or power drain on the control system that travels through the control network to reappear as some anomalous condition somewhere else in the system.
The second problem is associated with the single drive motor normally used to turn the controller drive shaft that, in turn, turns the valve stem. The introduction of power at only one point along the drive shaft produces transverse loads on the drive shaft and its bearings that result in rapid bearing wear and increased turning friction thus limiting the amount of turning torque developed in the controller. Further, different valves and/or processes require different stem turning speed (rpm) and different torque values. Accordingly, the manufacturer has need to maintain a large inventory of these special motors to avoid long delays in providing ready replacements. Such a situation raises the cost of each motor.
The third problem involves misalignment in the valve stem and valve actuator, either through vibration, ordinary wear of the parts or accidental bumping during normal maintenance operations. Even slight misalignment will generate frictional forces that oppose the rotation of the actuator and could result in incomplete actuation of the valve or, at the very least, increase the rate of wear of the actuator components.
To overcome the friction buildup problem, the prior art has suggested the use of coil springs arranged in a rather complicated fashion around the controller drive shaft to develop stored energy during drive shaft turning so as to release and provide additional energy when the controller begins its travel in opening the fully-closed valve. In U.S. Pat. No. 4,203,573 there is shown a coiled spring around the controller drive shaft to store energy as the drive shaft turns the valve stem from its opened to its closed position. When the motor is reversed to open the valve, the energy stored in the spring is released. While this teaching will provide a constant release of kinetic energy during valve travel, until the coil reaches its fully unwound position, there remains a transient counterforce applied to the valve actuator that may interfere with other phases of valve control. In U.S. Pat. No. 4,621,789 it is suggested to incorporate a coiled spring with a ratchet and pawl mechanism in a valve to be wound tight during operation of the valve for automatic release during power failure to reverse the valve position. The teaching, while suggesting the use of a coiled spring, is only for use in the event of power failure and otherwise will remain in a tightly wound configuration during operation of the actuator motor.
As to the second problem of single-point power input to the controller drive shaft, the prior art has suggested the use or more than one drive motor, however the use of these multiple motors is not for the purpose of turning the drive shaft solely in one direction. In U.S. Pat. No. 3,434,025 dual motors are used to reduce backlash in a control system by creating a drag torque in one motor to be overcome by the other motor during driving in one particular direction so that the teaching of two or more motors to provide direct drive input to the bull ring on the drive shaft is neither described nor suggested. In U.S. Pat. No. 3,231,803 the use of more than one drive motor for a drawworks is taught; however, the motors are specifically established far larger in size than would normally be required, and the field torque is altered through shunt means to eliminate the need for a variable speed transmission. In U.S. Pat. No. 741,995, a pair of motors is taught to be utilized in driving an electric capstan; however, there is no valve actuation involved in such a device and therefore there is no peak frictional load to be overcome during the beginning of movement of the capstan as there is in the instant matter.
As to the third problem, routine preventative maintenance procedures are employed to insure alignment of the actuator with the valve stem, however, this is time-consuming and adds to the cost of maintenance.